Stars and Brightness — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. A solar system is made up of one or more stars and the planets, moons, and other objects bound to them by gravity.
Gravity holds planets, moons, and asteroids in orbit around their central star.
2. Our solar system sits inside a spiral galaxy called the Milky Way.
The Milky Way is the spiral galaxy containing our Sun and roughly 100 to 400 billion other stars.
3. The neighboring spiral galaxy nearest to ours, visible from dark skies, is called the Andromeda Galaxy.
Andromeda is our closest large neighbor galaxy and is moving toward the Milky Way over billions of years.
4. A light-year measures distance, not time, in space.
Although the unit name includes year, a light-year measures distance traveled by light in one year.
5. Light takes about eight minutes to travel from the Sun to Earth.
Sunlight reaches Earth in roughly 8.3 minutes due to the Sun's relative closeness.
6. Light from Proxima Centauri takes about four years to reach Earth.
Because Proxima Centauri is about 4.2 light-years away, its light takes about 4.2 years to arrive.
7. Light from the Andromeda Galaxy takes about 2.5 million years to reach Earth.
Andromeda is about 2.5 million light-years away, so its light takes that long to reach us.
8. A galaxy contains many stars and many solar systems, all held together by gravity.
Galaxies are vast collections of stars, many of which host their own solar systems.
9. Astronomers use light-years instead of miles because space distances are huge.
Light-years let scientists describe enormous space distances with smaller, more practical numbers.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Sun to Earth
→ About 8 light-minutes
About 8 light-minutes
Sun to Proxima Centauri
→ About 4.2 light-years
About 4.2 light-years
Milky Way to Andromeda Galaxy
→ About 2.5 million light-years
About 2.5 million light-years
Earth to the Moon
→ About 1.3 light-seconds
About 1.3 light-seconds
Light travel times provide a clear way to compare the very different scales of distances in space.